Bowline

 
A good knot for forming a single loop that will not constrict the object it is placed around and is easily untied.
   

Step 1 - Form an overhand loop in the standing part.

   

Step 2 - Pass the working end upward through the loop.

   

Step 3 - Pass the working end over the top of the loop under the standing part.

   

Step 4 - Bring the working end completely around the standing part and down through the loop.

Step 5 - Pull the knot snugly into place, forming an inside bowline with the working end on the inside of the loop.